Tires Don't Have to be Worn Out to Lose Traction in Wet Weather, Study Shows

Car tires lose grip long before they wear out, but a new study from the Automobile Association of America (AAA) provides sobering information about just how dangerous older tires can become on rain-slickened roads.

The study released Thursday shows that driving on wet roads with tires that have only 4/32” of tread depth left could increase the stopping distance of a passenger car by up to 87 feet, and reduce a driver’s ability to control a vehicle by 33 percent.

“Even at that tread depth, we saw a significant decrease of the vehicle’s ability to stop in wet conditions,” says Greg Brannon, director of automotive engineering at AAA.

Vehicles braking at 60 mph with mostly worn tires were still traveling at 40 mph when they reached the point where vehicles with new tires had stopped, he pointed out.

All-season tires are considered worn at 2/32”. They typically start with between 10/32” and 11/32” of tread.

For the study, AAA tested new all-season tires against well-worn tires, with just 4/32” of tread depth remaining.

CR has also measured longer wet stopping distances as tread depth is diminished, according to Gene Petersen, CR’s tire program manager. “Keep in mind water depth, surface conditions, and many other factors define how well tires will stop,” he said.

“As tires wear, a tire’s ability to handle wet braking, resist hydroplaning, and have snow traction will decline,” Petersen said. “Dry braking and handling might actually improve when tires are worn, though clearly the negatives outweigh the positives for most driving situations.”

About 9 percent of vehicle crashes are tire-related, according to estimates from a study by the National Highway Traffic Safety Administration (NHTSA).

How can consumers know if their tires are dangerously worn? Both AAA and CR recommend that owners conduct the “quarter test”: Place an upside-down quarter in the grooves of a tire’s tread. If you can see the top of Washington’s head, it’s time to start shopping for new tires, because there’s only 4/32” of treadwear left.

“At 4/32" you still have some all-weather grip left, which will give owners time to research and shop for the best tire,” Petersen said.

Use the similar “penny test” to determine if your tires are worn to the point where they need to be replaced. Place a head-down penny into a major tire groove. If the the top of Lincoln's head is visible, then the tread is 2/32”, and the tire is worn out and needs to be replaced immediately. Tires also have treadwear indicators—platforms or "bars" sitting inside the major grooves that become flush when the tread wears down to 2/32".

Petersen says that manufacturers design tires to still function right down to the treadwear bars, making the penny test a valid method for determining when tires must be replaced. Still, drivers should keep an eye on their tires all along. “As the AAA study highlights, don't expect the worn tires to perform like new,” he said.

“Even with the most advanced driver assistance systems out there that are paving the way to autonomy, right now the systems don’t know the tread depth on your tires,” Brannon said. “It’s really incumbent on the driver to maintain the vehicle and drive appropriately for the conditions.”

In other words, a car’s advanced safety systems are only as effective as the tires the car is riding on.